Photodynamic therapy (PDT) is used as a non-surgical treatment of tumors. It combines non-toxic drugs and non-hazardous photosensitizing illumination, both innocuous agents by themselves, to generate, in the presence of molecular oxygen, cytotoxic reactive oxygen species (ROS) in situ which can kill or inactivate cells. Being a binary treatment modality, PDT allows for greater specificity and has the potential of being more selective, yet not less destructive, when compared with commonly used chemotherapy or radiotherapy.
Porphyrins have been employed as the first generation photosensitizing agents in clinics. Porfimer sodium (Photofrin®, a trademark of Axcan Pharma Inc.), the world's first approved photodynamic therapy agent, which is obtained from hematoporphyrin-IX by treatment with acids and has received FDA approval for treatment of esophageal and endobronchial non-small cell lung cancers, is a complex and inseparable mixture of monomers, dimers, and higher oligomers.
Large amount of work has been devoted to the synthesis of single pure compounds—so-called “second generation” sensitizers—which absorb at long wavelength, have well established structures and exhibit better differentiation between their retention in tumor cells and their retention in skin or other normal tissues. In order to optimize the performance of the porphyrin drugs in therapeutics and diagnostics, several porphyrin derivatives have been proposed in which, for example, there is a central metal atom (other than Mg) complexed to the four pyrrole rings, and/or the peripheral substituents of the pyrrole rings are modified and/or the macrocycle is dihydrogenated to chlorophyll derivatives (chlorins) or tetrahydrogenated to bacteriochlorophyll derivatives (bacteriochlorins).
Due to their intense absorption in favourable spectral regions (650-850 nm) and their ready elimination after treatment, chlorophyll (Chl) and bacteriochlorophyll (Bchl) derivatives have been identified as excellent sensitizers for PDT of tumors, having superior properties in comparison to porphyrins. Bacteriochlorophylls are of potential advantage compared to the chlorophylls because they show intense near-infrared bands, i.e., at considerably longer wavelengths than chlorophyll derivatives.
Bacteriochlorophylls are photosynthetic pigments that occur in various phototrophic bacteria. They are related to chlorophylls, which are the primary pigments in plants, algae, and cyanobacteria. Bacteria that contain bacteriochlorophyll (Bchl) conduct photosynthesis, but do not produce oxygen. They use wavelengths of light not absorbed by plants or Cyanobacteria. Different groups of bacteria produce different types of bacteriochlorophyll:
In vivo infraredabsorption maximumPigmentBacterial group(nm)BchlaPurple bacteria (Proteobacteria);805, 830-890Chloracidobacterium thermophilumBchl bPurple bacteria835-850, 1020-1040Bchl cGreen sulfur bacteria; Chloroflexi;745-755C. thermophilumBchl csChloroflexi740Bchl dGreen sulfur bacteria705-740Bchl eGreen sulfur bacteria719-726Bchl gHeliobacteria670, 788
Chemically, bacteriochlorophylls a, b, and g are bacteriochlorins, meaning their molecules have a bacteriochlorin macrocycle ring with two reduced pyrrole rings (B and D). Bacteriochlorophyll a (herein Bchla) is a compound of the formula:

Bacteriochlorophylls c, d and e are chlorins, meaning their molecules have a chlorin macrocycle ring with only one reduced pyrrole ring (D).
Purple Photosynthetic Bacteria
The purple photosynthetic bacteria are able to derive their cellular energy from light, organic compounds or inorganic compounds, depending on the chemical and physical environment. This remarkable versatility usually means that one mode of metabolism is utilized at a time so as to prevent unnecessary biosynthesis of alternative energy systems. Thus, photosynthetic metabolism and hence pigment biosynthesis only occurs under a limited set of conditions.
In regard to their ability to produce bacteriochlorophyll there appear to be two main groups within the purple bacteria. The first group includes Rhodobacter (Rba.) spheroidedes, Rba. Capsulatus and Rhodospirilum rubrum, which photosynthesize and thus produce bacteriochlorophyll anaerobically in the light; they can also synthesize significant amounts of pigment in the dark, but only under low aeration conditions. This indicates an oxygen and light dependent control over the genes encoding the enzymes involved in the synthesis of these pigments. The second group, which includes Rhodovulum sulfidophilum, sp. and Rubrivivax (Rvi.) gelatinosus, is able to synthesize pigments in the dark and to photosynthesize under both anaerobic and aerobic conditions.
The biosynthesis of Bchla in the facultative aerobic bacterium Rhodovulum sulfidophilum, formerly known as Rhodobacter sulfidophilus, was described by Porra et al., 1998 (Porra, R J, M. Urzinger, J. Winkler, C. Bubenzer, and H. Scheer, Eur. J. Biochem. 257, 185-191).
Bchla obtained from Rhodovulum sulfidophilum has served as the basis for further derivation and modification to produce improved Bchla derivatives employed in PDT and vascular-targeted PDT (VTP) of tumors and other pathological conditions as disclosed, for example, in U.S. Pat. No. 6,147,195, EP 863903, U.S. Pat. Nos. 6,333,319, 6,569,846, WO 2004/045492 and U.S. Pat. No. 8,207,154.
The use of Bchla derivatives for PDT and PDT research is increasing; therefore, there is a growing need to produce higher amounts of Bchla by fermentation.